The present invention relates to a system and method for monitoring a mechanical condition of a reciprocating compressor and, more particularly, to a non-invasive system and method that measures dynamic performance of a hyper-compressor cylinder by measuring the inherent operating strain of the compressor assembly.
The production of low-density polyethylene requires the use of very high pressures. Polymerization pressures can reach as high as 50,000 psi. To achieve these pressures, high pressure reciprocating compressors, or hyper-compressors, are used. Monitoring the mechanical condition of the hyper-compressor cylinder components during operation is important for determining maintenance requirements. That is, hyper-compressors are susceptible to similar problems as lower pressure reciprocating compressors, including, for example, valve failure, valve leakage, packing leaks and the like. The plungers used to compress the volatile gas are constructed of materials that have a high compressive strength but are brittle and typically will shatter when breaking. Valve failures can result in undue stress on the compressor running gear because of pressure unbalance or can result in loose pieces of the valve falling into the compressor chamber causing metal-to-metal contact on the plunger face that can cause bending and ultimate failure of the plunger. Such failure results in mechanical destruction of the compressor and in the volatile gases being released, which are susceptible to ignition, thereby causing a safety concern for those working in the vicinity of the compressor. Direct measurement of the high pressures to determine cylinder performance by penetrating the chamber is highly dangerous. Knowledge of the cylinder pressure gives insight into the valve and packing performance and the ability to avert many potentially catastrophic conditions.
The strain measurement of the compressor assembly has been done, as described in U.S. Pat. No. 7056,097, using strain gauges mounted on the head bolts or on the tie bolts, or compressive load measurements between the head and the head-bolt nut. The load measuring system used in the noted publication inserts an apparatus as part of the load bearing structure and is subject to deformation or crushing, which alone could be a safety concern. Externally mounted strain gauges require cleaning of the surface to support a good bond to the stressed element. Additionally, such strain gauges are temperature dependent. Moreover, gauges are small and require protection to survive in an industrial environment and are cyclically stressed when installed on an operating compressor. Clamp-on strain assemblies have been used, but they also require cleaning of the surface, and since the contact area is under stress, the contact surfaces will “creep” over time and under cyclic stress.